1. Field of the Invention
The present invention relates to an apparatus for setting heating conditions in a heating furnace and a thermal analyzer for an object to be heated in the heating furnace. More particularly, the present invention relates to an apparatus for setting heating conditions which is to be suitably used for control of a reflow furnace for heating a printed-wiring board to perform soldering, for example, which is to be suitably used when heating the printed-wiring board in the reflow furnace, and a thermal analyzer for the printed-wiring board which is suitably used when setting the heating conditions.
2. Description of the Related Art
A printed-wiring board having electronic components mounted thereon (which is also referred to as a circuit substrate, a printed board, a printed substrate or the like) is usually manufactured by a process of printing a creamy soldering material (soldering paste) onto the printed-wiring board, mounting the electronic components thereon, and putting them into a reflow furnace (which is also referred to as a reflow soldering apparatus or a reflow apparatus) to perform reflow soldering.
In this process, the printed-wiring board is put on a conveyer provided in a tunnel-shaped furnace and is carried. At the carrying step, the printed-wiring board is heated by a heating source provided in the furnace, the soldering paste is melted with an increase in a temperature of the printed-wiring board, is then carried to the outside of the furnace and exposed to room temperature for cooling. Consequently, the solder is solidified. Thus, soldering is completed.
In the reflow soldering process, the printed-wiring board is heated to a temperature of 200.degree. C. or more at which the solder is melted. In that case, heating control is performed in such a manner that a temperature profile becomes a curve shown in a graph of FIG. 5. In order to lessen thermal damages caused by rapid heating, a temperature is not simply raised but preheating is performed. Then, uniform heating is performed to hold the temperature of the printed-wiring board constant. Thereafter, heating is performed to a temperature of 200.degree. C. or more at which the solder is melted. Thus, heating control is performed in three stages.
However, a plurality of components having various heat capacities are generally mounted on the printed-wiring board so that the temperature is varied on the printed-wiring board. Therefore, it is very hard to set heating conditions that the printed-wiring board is heated enough to melt the solder and that temperatures of small components are not raised too much. For this reason, a plurality of heating sources having different properties are provided in the reflow furnace to vary the influence of the heating source depending on a position of the printed-wiring board. More specifically, a plurality of near-infrared radiation heaters and a plurality of far-infrared radiation heaters or a plurality of hot air fan heaters whose temperatures are kept different are provided above and below the conveyer so that a heating zone is divided into plural portions. Heating conditions of the heating source in each heating zone are properly set respectively to heat the printed-wiring board in such a manner that a temperature difference of the printed-wiring board is reduced as much as possible and a temperature distribution of the printed-wiring board is uniform.
For this reason, conventionally, a temperature sensor such as a thermoelectric couple has been attached to the printed-wiring board to measure a rise in the temperature of the printed-wiring board, and temperature setting has been changed repeatedly until a target temperature profile is obtained.
However, a work for changing the temperature setting is performed by a worker's perception or guess. Therefore, there is no assurance that optimum heating conditions can be set. Furthermore, the heating conditions are repeatedly set and the temperature of the printed-wiring board is repeatedly measured to find the optimum conditions. Consequently, plenty of time is required to set the optimum heating conditions, and the worker's skill and experience are necessary.
In a case where the heating conditions in the reflow furnace are not suitable, for example, the printed-wiring board is heated too much, a thermal stress is easily applied to small chip components having small thermal capacities and the printed-wiring board. Conversely, in a case where the heating is insufficient, solder joint portions of large components having great thermal capacities are unmelted.
For a printed-wiring board to be newly manufactured, therefore, it is necessary to find optimum heating operation conditions for the reflow furnace in which a thermal stress applied to the components and the printed-wiring board is lessened as much as possible and solder joint portions can fully be heated. For each solder joint portion on the printed-wiring board, a temperature profile should be examined in detail.
This examination is performed by repeatedly, several times, acquiring the temperature profile of the printed-wiring board (by measuring the same temperature profile by means of a thermoelectric couple) and resetting heating conditions such as a heater temperature and the like. For this reason, a lot of man-day is required to fix the thermoelectric couple and to measure the temperature profile plural times. Furthermore, every time the heating conditions are changed, plenty of time is taken to stabilize the heater temperature, that is, considerable man-day and time are required. In addition, the worker's experience and accumulation of know-how are important in order to predict the optimum heating conditions based on a result of the measurement of the temperature profile.
Therefore, the following has been investigated. Thermal analysis is performed by using a computer to quantitatively grasp a heating state in the reflow furnace in order to enhance reliability of the printed-wiring board. Moreover, the temperature profile of a heated object in the reflow furnace is predicted and utilized for setting the optimum operation conditions in the reflow furnace.
FIG. 10 is a flowchart showing a processing of performing thermal analysis for the printed-wiring board in the reflow furnace using the prior art.
At Steps 501 to 507, an analytic model of the printed-wiring board is first generated in order to perform the thermal analysis for the printed-wiring board. On the printed-wiring board are mounted electronic components having several hundred or more junction terminals such as lead frames having fine and complicated shapes which are referred to as a QFP (Quad Flat Package), a SOP (Small Outline Package), a BGA (Ball Grid Array) and the like.
At the Step 501, a method for simplifying such an analytic model of the electronic component is examined in order to shorten a computation time or to reduce man-day for creating the analytic model. It is necessary to simplify the analytic model without reducing computation precision. To perform the simplification, technical knowledge and experience are required.
Next, the analytic model of the component is created at Step 502 based on a result of the examination of the simplifying method (the Step 501) At Step 503, a physical property value is defined for the created analytic model of the component.
In general, a plurality of electronic components are mounted on the printed-wiring board. Therefore, a work for simplifying the analytic model of the component (the Step 501) and a shape forming work (the Step 502) are performed for each component (Step 504).
At the end of the work for creating the analytic model, the shape of the printed-wiring board for mounting the electronic components thereon is formed, a physical property is defined (Steps 505 and 506), and a final analytic model of the printed-wiring board is created.
In a case where the printed-wiring board has a large size and a number of electronic components are mounted on the printed-wiring board, the work for creating the analytic models of the electronic components requires a lot of man-day. In a case where analysis is utilized for designing the printed-wiring board, a work which requires such enormous man-day should be repeated every time a layout of the components is changed. In a case where analysis is performed to set the operation conditions for the reflow furnace on a manufacturing site or the like, a plurality of shape models of the printed-wiring board should be created and analyzed in a time which is as short as possible.
At the Step 507, a computation grid is created on the analytic model manually or automatically. More specifically, the analytic model is divided into grids in proper positions.
As described above, a reflow furnace using an infrared heater has frequently been utilized for the soldering of the electronic components onto the printed-wiring board because the reflow furnace has excellent productivity. A plurality of infrared heaters having temperatures variously set are arranged on upper and lower faces of the reflow furnace. The printed-wiring board is carried for heating by means of a conveyer at a constant speed between the infrared heaters.
Accordingly, it is necessary to accurately calculate a radiation heat quantity received by the printed-wiring board from the infrared heater at Step 508 in order to perform the thermal analysis for the printed-wiring board in the reflow furnace. The Step 508 will be described below.
A net radiation heat quantity received by the printed-wiring board is calculated based on a balance of the radiation heat quantity received by the printed-wiring board and a radiation heat quantity. The heat quantity received by the printed-wiring board is obtained from a product of a radiation heat quantity of each heater which is calculated based on a surface temperature of each heater arranged in the furnace and an emissivity of a surface of the heater and a ratio (Radiation shape factor) in which the radiation heat quantity reaches the printed-wiring board from the heater.
Examples of the heat quantity received by the printed-wiring board include a radiation from an internal wall of the furnace to the printed-wiring board, a radiation which is reflected on a peripheral wall and reaches the printed-wiring board from the heater, and the like. These heat quantities are also calculated. The net radiation heat quantity received by the printed-wiring board is a difference between the radiation heat quantity received by the printed-wiring board and a radiation heat quantity emitted from the printed-wiring board toward the outside.
The printed-wiring board receives a heat quantity by convection heat transfer in an atmosphere in the furnace and is thus heated. Therefore, the heat quantity is also calculated at Step 509. The sum of these heat quantities is set as a boundary condition to an analytic model of the printed-wiring board at Step 510.
Next, the printed-wiring board having the boundary condition set is analyzed at Step 511. This analysis is executed by a finite element method, a difference method and the like. As described above, the printed-wiring board is carried by means of the conveyer in the heating furnace, and a relative position relationship between each heater and the printed-wiring board is changed with time. For this reason, the radiation heat quantity received by the printed-wiring board, that is, the radiation boundary condition is not constant but is changed with time. Therefore, it is necessary to recalculate the radiation boundary condition every certain time. Also in a heating state obtained by the convection, an atmospheric temperature and a convection heat transfer coefficient are varied depending on a position in the furnace. Therefore, it is also necessary to recalculate the convection boundary condition every certain time. For this reason, it is decided whether or not the analysis has been performed up to an outlet of the furnace at Step 512. If the outlet of the furnace is not reached, the Steps 508 to 511 are repeated.
With conventional thermal analysis software, such a boundary condition which is changed with time has been neither calculated nor automatically set to a thermal analysis object. Therefore, it has been necessary to manually perform all these works.
Thus, the analysis is completed. Analysis precision is sometimes low depending on the method for simplifying the analytic model of a component at the Step 501. In some cases, therefore, it is necessary to reconsider the simplifying method at Step 514 and to perform the analysis from the beginning again.
Conventionally, the analytic model of the printed-wiring board created by using much man-day has been set by repetitive calculation of the radiation boundary condition and the convection boundary condition in each position in the furnace as described above, thereby performing thermal analysis while the printed-wiring board enters the reflow furnace and gets out thereof.
Thus, the conventional thermal analyzing technique has the following problems. Therefore, the thermal analysis for the printed-wiring board in the reflow furnace could not easily be performed.
1+L The thermal analysis object (printed-wiring board) is carried by means of the conveyer in the heating furnace, and radiation and convection boundary conditions are changed with time. Therefore, it is necessary to calculate and set the radiation boundary condition and the convection boundary condition for the thermal analysis object every constant time.
2+L Much complicated man-day is required to form the shape of the analytic model of the printed-wiring board. In addition, technical knowledge and experience are required to simplify the analytic model of the electronic component mounted on the printed-wiring board.
In order to solve these respects, accordingly, it is important to develop the method for simplifying the analytic model having high analysis precision which can be applied to non-steady heat conduction analysis (for example, thermal analysis for the printed-wiring board moving in the reflow furnace) for a long time.
FIG. 17 shows a state of an actual printed-wiring board seen from a side. Various components are mounted on a substrate 601. The reference numeral 602 denotes a SOP (Small Outline Package), the reference numeral 603 denotes a QFP (Quad Flat Package), and the reference numeral 604 denotes a PGA (Pin Grid Array). Each component is connected electrically and mechanically to the substrate 601 through a lead 605 and a pin 606 of a connecting terminal.
In a case where the heat conduction analysis is performed for the printed-wiring board by using the difference method or the like, it is necessary to perform a work for modeling (simplifying) the shape of the printed-wiring board to create a computation grid on the shape. An analytic model having a computation grid created as shown in FIG. 18 is generally taken as the printed-wiring board having the shape shown in FIG. 17. FIG. 18 shows a state of the printed-wiring board seen from a side as in FIG. 17. For explanation, the computation grid is shown in two dimensions. The actual printed-wiring board has a three-dimensional shape. Correspondingly, the computation grid is also created in three-dimensions.
A computation time necessary for the heat conduction analysis or the like is proportional to the number of computation elements, and is inversely proportional to a square of a space between the computation grids. For this reason, in a case where the computation grid is created as shown in FIG. 18, the number of three-dimensional computation elements is enormous. In addition, since the computation grid is created according to a fine lead section and components having various heights, the space between the computation grids becomes small. Therefore, the computation time is increased enormously in FIG. 18.
In order to shorten the computation time, it is necessary to increase the computation element. In a case where a printed-wiring board having a complicated shape is analyzed by using the difference method, the number of the computation elements is large as shown in FIG. 18 and a dimension of the computation element is reduced. Therefore, the computation could not conventionally be performed in a short time.
In order to solve such a problem of the computation time, conventionally, the printed-wiring board has generally been treated as a two-dimensional analytic model to shorten the computation time. In this case, however, electronic components mounted on the substrate are simply modeled. Therefore, heat transfer between the substrate and the electronic component cannot accurately be considered for the analysis so that errors are increased around the components.